Previous studies have identified the link between chlorine and asthma, but the new findings published in the European Respiratory Journal cast doubt on the idea that outdoor pools are safer than indoor ones because chlorine vapours don't remain trapped inside an enclosed space.

"What is new in this study is that we looked at outdoor pools for the first time," says Professor Alfred Bernard, a toxicologist at Catholic University of Louvain in Brussels, who led the study.

Asthma, which affects more than 300 million people worldwide, is the most common paediatric chronic illness. Symptoms include wheezing, shortness of breath, coughing and chest tightness.

Greater exposure

Bernard and colleagues found that children tend to spend more time in outdoor pools.

As a result children are more likely to swallow chlorinated water or ingest vapours, which contain chemicals that attack the cellular barriers protecting the lung from allergens, says Bernard.

"We see that the risk of the outdoor pool is equal and even higher than indoor pools because children tend to spend longer in outdoor pools and they are more chlorinated," he says. "The more you swim, the higher the risk."

The Belgian team tested 847 students around the age of 15 years for allergies and asthma and asked their parents about exposure to asthma risks such as tobacco smoke, pets and pollution, and how much time the children had spent in chlorinated pools.

The researchers determined that the risk for children predisposed to allergies and asthma was directly related to the amount of time spent in a pool.

Children with the highest pool attendance - one hour per week for 10 years - were five times more likely to be asthmatic than young people who had never swum in a pool, the study found.

"Young children are more exposed because they take more water into their airways and their lungs are still developing," says Bernard.